After being turned away for a second time from the Hoover Dam in 2281 Caesar's Legion focused it's efforts eastward. Never the less it was obvious that in the future there would be further conflict between the Legion and the New California Republic and it became increasingly obvious that while simple lever action rifles had sufficed as a standard infantry arm against the various tribes that it had previously fought they were ill suited for use against the comparatively advanced weapons employed by the NCR army. The Polybolos was the primary product of said efforts. Chambered to 5.56mm ammunition the Polybolos's main advantage was that it was simple to produce and maintain, even in armories manned mostly by untrained slaves and by uneducated legionaries. Developed in 2283 by 2285 it had managed to supersede the "cowboy repeaters" that had previously been the mainstay of the Legion's arsenal, though these would be retained for use by marksmen as well as training purposes.

The Polybolos was a blowback based fully automatic weapon. This required some adjustments among common legionaries used to slow firing single shot weapons. It's length was often an encumbrance, though many legionaries also found that this was advantageous in bayonet fights. Reliability was often an issue as the weapon needed frequent maintenance to stay in top condition. Quality gradually improved over a period of a decade after the first weapons entering service, though there was some variation between weapons made in different armories. In a few border skirmishes between Legion and NCR troops the Polybolos proved to be serviceable enough even if after capturing a few of these weapons the NCR army officially deemed that it was inferior to their service rifles. As the Legion's forces marched out into the Great Plains and the scattered settlements there-in the Polybolos Assault rifle proved devastating.